Robust Nitrogen-Doped Microporous Carbon via Crown Ether-Functionalized Benzoxazine-Linked Porous Organic Polymers for Enhanced CO2 Adsorption and Supercapacitor Applications

Nitrogen-doped carbon materials, characterized by abundant microporous and nitrogen functionalities, exhibit significant potential for carbon dioxide capture and supercapacitors. In this study, a class of porous organic polymer (POP) were successfully synthesized by linking Cr-TPA-4BZ-Br4 and tetraethynylpyrene (Py-T). The model benzoxazine monomers of Cr-TPA-4BZ and Cr-TPA-4BZ-Br4 were synthesized using the traditional three-step method [involving CH=N formation, reduction by NaBH4, and Mannich condensation]. Subsequently, the Sonogashira coupling reaction connected the Cr-TPA-4BZ-Br4 and Py-T monomers, forming Cr-TPA-4BZ-Py-POP. The successful synthesis of Cr-TPA-4BZ-Br4 and Cr-TPA-4BZ-Py-POP was confirmed through various analytical techniques. After verifying the successful synthesis of Cr-TPA-4BZ-Py-POP, carbonization and KOH activation procedures were conducted. These crucial steps led to the formation of poly(Cr-TPA-4BZ-Py-POP)-800, a carbon material with a structure akin to graphite. In practical applications, poly(Cr-TPA-4BZ-Py-POP)-800 exhibited a noteworthy CO2 adsorption capacity of 4.4 mmol/g, along with specific capacitance values of 397.2 and 159.2 F g–1 at 0.5 A g–1 (measured in a three-electrode cell) and 1 A g–1 (measured in a symmetric coin cell), respectively. These exceptional dual capabilities stem from the optimal ratio of heteroatom doping. The outstanding performance of poly(Cr-TPA-4BZ-Py-POP)-800 microporous carbon holds significant promise for addressing contemporary energy and environmental challenges, making substantial contributions to both sectors.


Synthesis of trans-Di(nitrobenzo)[18]crown-6 (Dibenzo-crownether-2NO2) [Cr-2NO2] 1
A solution of dibenzo[18]crown-6 (3.00 g, 9.23 mmol) in a mixture of CHCl3 (80 mL) and acetic acid (10 mL) in a 250-mL two-neck flask was stirred at room temperature for 1 h.A mixture of acetic acid (5 mL) and HNO3 (2 mL) was added dropwise and then the mixture was heated at 50 °C for 24 h.The white solid was filtered off and washed with MeOH to afford the title compound

Synthesis of Dibenzo-crownether-4NO2 [Cr-TPA-4NO2] 2
The mixture of Cr-2NH2 (1 g, 2.56 mmol), 1-fluoro-4-nitrobenzene (1.63 mL, 15.38 mmol), K2CO3 (6.37 g, 46.09 mmol), Cu (0.033 g, 0.52 mmol) and DMF (30 mL) was heated and stirred under N2 at 110 °C for 24 hours.Following the reaction, the mixture underwent filtration to remove the solid, and subsequently, the DMF solvent was evaporated under reduced pressure, resulting in the formation of a viscous brown solid.The brown solid was then dissolved in the DCM and added dropwise into the methanol to obtain Cr-TPA-4NO2 as the brown powder.

Synthesis of Dibenzo-crownether-4NO2 [Cr-TPA-4NH2] 2
The mixture of Cr-TPA-4NO2 (1 g, 1.14 mmol), Pd/C (1.63 mL, 6.86 mmol), DO (40 mL), and EtOH (20 mL) was heated and stirred under N2 at 90 °C for 1 hour.After one hour, a gentle addition of 1.89 mL (38.96 mmol) of NH2NH2.H2O was made to the mixture, which was subsequently heated at 90 °C for two days.The resulting mixture was subjected to filtration to eliminate any unreacted Pd/C, followed by evaporation of the solvent under reduced pressure.This process led to the formation of Cr-TPA-4NH2 as a white powder.

The experimental details for DFT calculations
Density functional theory (DFT) calculations were conducted at the B3LYP/6-31G(d) level using the Gaussian 09W program.The consideration of the D3BJ dispersion correction was essential to address long-range and non-covalent interactions effectively.Moreover, the global minimum of each conformer in the ground-state geometry was identified through harmonic vibrational frequency analysis to determine the lowest energy conformer for subsequent analysis.The calculations encompassed the determination of the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and molecular electrostatic potential (MESP) at optimized geometries utilizing the same level of theory.

Electrochemical Analysis
Working Electrode Cleaning: Before using, the glassy carbon electrode (GCE) was polished several times with 0.05-µm alumina powder, washed with EtOH after each polishing step, cleaned through sonication (5 min) in a water bath, washed with EtOH, and then dried in the oven at 50 o C.

Electrochemical Characterization:
The electrochemical experiments were performed in a threeelectrode cell using an Autolab potentiostat (PGSTAT204) and 1 M KOH as the aqueous electrolyte.The GCE was used as the working electrode (diameter: 5.61 mm; 0.2475 cm 2 ); a Pt wire was used as the counter electrode; Hg/HgO (RE-1B, BAS) was the reference electrode.All reported potentials refer to the Hg/HgO potential.A slurry was prepared by dispersing Cr-TPA-4BZ-Py-POP or poly(Cr-TPA-4BZ-Py-POP)-800 (2 mg), carbon black (2 mg), and Nafion (10 wt%) in a mixture of (EtOH/ H2O) (200 µL: 800 µL) and then sonicating for 1 h.A portion of this S6 slurry (10 µL) was pipetted onto the tip of the electrode, which was then dried in air for 30 min before use.The electrochemical performance was studied through CV at various sweep rates (5-200 mV s -1 ) and through the GCD method in the potential range from 0 to -1.00 V (vs.Hg/HgO) at various current densities (0.5-20 A g -1 ) in 1 M KOH as the aqueous electrolyte solution.
The specific capacitance was calculated from the GCD data using the equation.

Cs = (I∆t)/(m∆V)
Where Cs (F g -1 ) is the specific capacitance of the supercapacitor, I (A) is the discharge current, ΔV (V) is the potential window, Δt (s) is the discharge time, and m (g) is the mass of the NPC on the electrode.The energy density (E, W h kg -1 ) and power density (P, W kg -1 ) were calculated using the equations.

Electrochemical Analysis in Two-Electrode Symmetric Supercapacitor System
The slurry prepared by mixing poly(Cr-TPA-4BZ-Py-POP)-800, carbon black, and Nafion (10 wt.%) was coated onto a flexible Kuraray carbon paper (0.1 mm in thickness) with an effective area of 1 cm × 1 cm and then dried at 100 °C overnight in a vacuum oven.The mass loading of active material on the current collector was 0.8 mg cm -2 .The two working electrodes were separated with filter paper and infiltrated with potassium hydroxide (1 M) aqueous solution.
The specific capacitance was calculated from galvanostatic charge-discharge experiments using the following equation:

S7
Where Cs (F/g) is specific capacitance of the supercapacitor, I (A) is the discharge current, ΔV (V) is the potential window, Δt (s) is the time, and m (g) is the mass of porous carbon on the one electrode.The energy density (E, Wh kg -1 ) and power density (P, W kg -1 ) were calculated using the equations.S2.The specific capacitance of Cr-TPA-4BZ-Py-POP from CV profiles at different scan rates (three-electrode system).